Trematodes (Flukes)

Trematodes are also referred to as flatworms. Trematode life cycle: – Snails are intermediate hosts and that this is where asexual reproduction occurs. – Some trematodes have second intermediate hosts, such crabs or fish. – Larval forms infect humans and mature to adult forms. – The adult worms lay eggs that are then released into the environment. Worm migration is associated with fever, chills, and eosinophilia. In most cases, the worm burden is low and patients are asymptomatic or have only mild symptoms. In this tutorial, we'll learn the consequences of more serious infections. Schistosoma japonicum and Schistosoma mansoni cause intestinal schistosomiasis. Acute form manifests as Katayama fever, which is characterized by gastrointestinal symptoms, pain, cough, enlargement of the spleen and/or liver, and eosinophilia. If infection is chronic, granulomas can form around the eggs. Fibrosis, called Symmer's pipe stem peri-portal fibrosis (aka, clay pipe fibrosis) can lead to tissue destruction and portal hypertension. Granulomas can also form in other tissues, including the central nervous system. Schistosoma haematobium causes urinary schistosomiasis, also known as vesicular schistosomiasis. Hematuria, scarring, and calcification. Associated with urinary bladder cancer. Be aware that other species that parasitize birds and other mammals can cause cercarial dermatitis in humans. Schistosoma Life Cycle Be aware that trematodes have multiple larval stages with unique forms and names; for simplicity, we'll just refer to each of these forms as "larvae." More detailed version. 1. Free-living larvae invade snails (the intermediate hosts). 2. Within the snail, the larvae replicate and give rise to infective forms that exit the snail to seek their next host. 3. Upon contact with a human, the parasites penetrate the skin and migrate to the liver. 4. The parasite matures to its adult form in the portal blood. Schistosomes have separate sexes: the male is larger, and the "threadlike" female resides in his gynecophoric canal. 5. Paired Schistosomes migrate and lay eggs: Schistosoma japonicum and Schistosoma mansoni tend to lay eggs in the mesenteric veins, which causes intestinal schistosomiasis. – Schistosoma haematobium tends to lay eggs in the urinary venous plexus, which causes urinary schistosomiasis. 6. Eggs are excreted in the feces or urine, depending on their location. 7. In the environment, the eggs hatch and give rise to the larval forms that will invade snails. Be aware that not all eggs are excreted, but, instead, can become encased in inflammatory granulomas as described above. Schistosomiasis is also called "snail fever" and "Bilharziasis" Intestinal fluke: Fasciolopsis buski The largest intestinal fluke to infect humans. Humans ingest it when eating aquatic plants, particularly water chestnuts. Infection can cause gastrointestinal problems, fever, abdominal swelling, and intestinal obstruction. Liver flukes: Fasciola hepatica (sheep liver fluke) and Clonorchis sinensis (Chinese liver fluke) Fasciola hepatica is ingested with aquatic plants, particularly watercress. Infection can produce pain in the upper right quadrant, hepatic enlargement, and, in serious cases, liver rot (necrosis) and portal cirrhosis. Clonorchis sinensis is ingested with raw or undercooked freshwater fish. Infection can produce fever, pain, gastrointestinal problems, and inflammation of the biliary and pancreatic structures. Chronic inflammation is associated with cholangiocarcinoma (biliary duct cancer). Lung fluke: Paragonimus westermani Ingested with raw or undercooked crustaceans, such as crabs. Early infection can produce gastrointestinal symptoms and fever. Later, pulmonary symptoms can develop, and include cough with blood, chest pain, cavitation, bronchitis, fibrosis, and other compilations that impair respiration. These worms can also invade the central nervous system (cerebral paragonimiasis) and other organs. See links above for species-specific details. 1. Larval forms invade snails. Exception to this is Clonorchis sinensis, because snails eat their eggs. 2. Larvae replicate and exit the snail. 3. Depending on the trematode species, the parasites find their secondary hosts, such as crabs or fish, or they take up residence on aquatic plants. In these locations, they encyst, and await ingestion by a human or other animal host. 4. Parasites are ingested with animals or plants. 5. After ingestion, they excyst in the human small intestine. Immature flukes migrate to the lungs, biliary ducts, or small intestine, depending on the species. At these sites, the immature flukes develop to their adult forms, then self-fertilize and lay eggs. 6. Eggs are released in the feces (and sputum, in the case of lung flukes). 7. In the external environment, the eggs release the larvae that will invade snails and continue the cycle. Except for Clonorchis sinensis, whose eggs are eaten by snails. Blood flukes enter through the skin, whereas the tissue flukes are ingested with animals or plants. The blood flukes have separate sexes, while the tissue flukes are hermaphroditic (they self-fertilize).